Question

The beginning of a river is called the _____. mouth source watershed tributary

Answer 1

The correct answer is "source" - this is the beginning of the river. (typically in the mountains)

Mouth of a river is the "end" of the river- it's when the river joins the ocean.
A tributary is another river that ends in the main river
and the watershed is a piece of land that separates bodies of water.

Answer 2

The beginning of a river is called the source.